1. Field of the Invention
The present invention generally relates to improvements in a hydraulic control apparatus applied to a hydraulic servo system of an automatic transmission.
2. Description of the Prior Art
Japanese Patent Provisional Publication No. 60-109647 discloses a hydraulic control apparatus for an automatic transmission by which a hydraulic pressure supplied to a hydraulic servo system is controlled. As shown in FIG. 2, a pressure control valve 52 is arranged to implement a hydraulic control operation by using a hydraulic pressure of a hydraulic power source 54 supplied through a shift valve 53. The controlled hydraulic pressure by the pressure control valve 52 is supplied to a hydraulic servo system 56. An operation chamber 50a of an accumulator 50 is fluidly communicated with the pressure control valve 52 through a passage 58 and an orifice 60. The hydraulic pressure of the operation chamber 50a is fedback to the pressure control valve 52 so as to increase the controlled pressure value. On the other hand, the hydraulic pressure of the hydraulic servo system or apparatus 56 is fedback to the pressure control valve 52 so as to decrease the controlled pressure value. Further, the hydraulic pressure of the hydraulic power source 54 is applied to the pressure control valve 52 so as to increase the controlled pressure value.
However, because such a conventional hydraulic pressure control apparatus functions to always apply the hydraulic pressure of the hydraulic source 54 to the pressure control valve 52 in one direction, the characteristic of the accumulator 50 are changed by the change of a line pressure. That is, a build-up time period, in which the hydraulic pressure of the line is generally kept constant, is effected by the characteristics of the accumulator 50. Such a change of the build-up time period is represented by the following equations:
In FIG. 2, when various values are defined as follows:
Q is flow rate passing through an orifice 60, PA1 .alpha.: constant, PA1 a: passage cross-sectional area of the orifice 60, PA1 P.sub.S : hydraulic pressure of the hydraulic servo apparatus 56, PA1 P: hydraulic pressure in the operation chamber 50a, PA1 A.sub.1 : area of the operation chamber 50a, PA1 F: force applied to accumulator 50, PA1 A.sub.2 : pressure receiving area to a spring side of the pressure control valve 52, PA1 A.sub.3 : source hydraulic pressure working area of the pressure control valve 52, PA1 F.sub.S : spring force of the pressure control valve 52, PA1 P.sub.L : line pressure, PA1 x: stroke of the accumulator 50, and PA1 k: constant,
the following equations (1) to (5) are obtained: ##EQU1## By combining the above equations, the following equation (6) is obtained: ##EQU2## The equation (6) can be deformed as follows: ##EQU3## where P'=P.sub.L +F.sub.S /A.sub.3.
When t=0.fwdarw.t, it is defined that F=F.sub.1 .fwdarw.F and P=P.sub.1 .fwdarw.P. Next, the integration of the equation (7) is obtained, and by applying the interval [0, t] relative to t and the interval [F.sub.1, F] relative to F to the integration of the equation (7), the following equation (8) is obtained. EQU P=P'-(P'-F.sub.1 /A.sub.1) (1-t/t.sub.0).sup.2 ( 8)
where ##EQU4##
Defining that the accumulator 50 finishes the position change from 0 to x when the time T elapsed from the turning-on of the shift valve 53 (t=0.fwdarw.T), and that at this time P=P2, F=F2 then, EQU P.sub.2 =F.sub.2 /A.sub.1.
Accordingly, the following equation (9) is obtained from the equation (8). ##EQU5##
When the volume stroked in the accumulator 50 is V, the following equation (10) is obtained. EQU V=A.sub.1 (F.sub.2 -F.sub.1)/k (10)
Therefore, the following equation (11) is obtained. ##EQU6##
As is clear from the equation (11), the time T is varied according to the change of P'. Since P' changes according to the variation of the line pressure P.sub.L, the time T changes according to the line pressure P.sub.L. This indicates that the delay time by the conventional pressure control apparatus is effected by the line pressure P.sub.L.